Graduation date: 2006Previous observations of light levels and phytoplankton abundances along the Oregon coast demonstrated that phytoplankton attenuated light sufficiently to potentially limit the growth of intertidal macrophytes and therefore structure local intertidal communities. Inspired by this observation, in spring 2004, I initiated a study to quantify the direct and indirect benthic community response to different light regimes. Frames supporting plastic mesh were installed to shade macroalgae in the Hedophyllum zone. With unshaded treatments, the shade treatment manipulated the light levels available to benthic algal species at two sites differing in historic pelagic productivity. Treatments included shaded, unshaded, and manipulation control plots that were 0.25 m2 in area. The experiment began in mid June and ran through the end of August, 2004. The direct response variables were the growth rate and abundance accumulation of the perennial intertidal kelp Hedophyllum sessile. The indirect responses measured were the changes in abundance of understory red algae. Results indicated that shade decreased growth rates of Hedophyllum and that the extent of growth rate reduction varied between sites. The response of total cover of understory algae was not strongly different between treatments and the response of individual algal groups differed between treatments and between sites. At Fogarty Creek, an area of historically high macroalgal abundance and comparatively low phytoplankton abundance, the primary response to increased shade was an increase in foliose red algae. At Strawberry Hill, an area of lower macroalgal abundance, the primary response was an increase in dominance of coralline algae. The response of algal diversity to different canopy treatments also differed between sites. Diversity of understory algae was highest beneath the artificial shades at Fogarty Creek whereas at Strawberry Hill, the diversity was the highest beneath intact canopy. The difference in responses of the understory assemblage at the two sites was interpreted to signify potential differences in the ecological role of the canopy. At Fogarty Creek, the canopy appears to reduce algal abundance and diversity, presumably through competitive inhibition, whereas at Strawberry Hill the canopy appears to facilitate increased algal abundance and diversity, presumably through habitat amelioration. While underscoring the need to include parameters of environmental stress when modeling changes in total production, these results are consistent with estuarine studies that demonstrate that as nutrient loading increases, the community shifts from a mixed autotroph assemblage to one dominated by phytoplankton. Thus in open coast systems where annual benthic production can exceed the pelagic production by a factor of 5-10, increased nutrients may also shift these systems towards greater dominance by phytoplankton and reduced macrophytes. As perennial macrophytes such as kelp and sea grasses are also extraordinarily important habitat modifiers and provide nursery habitat for several species of fish and invertebrates, large-scale reduction of macrophytes could also lead to profound modifications of coastal ecosystem dynamics.